Only 20 percent of the clothes in the average person's closet are worn regularly. You can turn the other 80 percent of that clutter into cash.

Digital consignment shops, such as Poshmark and thredUP, offer you a simple way to sell your gently used clothing.

Users of the Poshmark app currently share more than 7 million items a day, said Manish Chandra, the company's founder and CEO. Thredup offers merchandise from 35,000 brands that people can buy and sell online.

"The right service for you comes down to whether your priority is ease-of-use or making the most money," said Trae Bodge, a shopping expert who has made hundreds of dollars over the past two years selling her old clothes online.

For a website like ThredUp, you don't have to work as much to sell your clothes. You order a bag for $9.99 and fill it up. (The company plans to eliminate the bag fee for all shoppers by the end of June.)

ThredUP decides what it wants to pay you for each item and discards the rest. You pay a $10.99 fee in advance if you want thredUP to send you the items the company doesn't want back to you.

"The time-consuming part of ThredUP was that waiting. It took eight weeks to get the total for my bag," said CNBC's retail reporter Courtney Reagan who tested out the service as well as Poshmark. (See the detailed results of her test below.)

You will have to work harder to move the merchandise on Poshmark, Bodge said. You have to create a listing for each item, take a picture of it and set the price. The company takes a $2.95 commission for sales under $15 and a 20 percent commission for sales over that.